[Bug 225341] System locks up due to atkbd_timeout kludge when using /dev/atkbd0 directly

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=225341

            Bug ID: 225341
           Summary: System locks up due to atkbd_timeout kludge when using
                    /dev/atkbd0 directly
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: jan.kokemueller at gmail.com

There is a timer in atkbd that is called 10 times per second. It seems to be a
kludge that is used to unwedge keyboard interrupts [1].

Unfortunately, if I keep this in, my system locks up after a brief period of
usage. I don't really know how to debug this -- the system just hangs.

One important bit of information: I don't use the atkbd device in its default
configuration behind the kbdmux0 device. Instead, I detach it from kbdmux0,
open it directly and parse the keyboard data in userspace [2]. I feed the data
into an Evdev device and a vkbd(4) device for use in vt(4).

In the default configuration (atkbd0 attached to kbdmux0) the issue is masked.

I haven't noticed any negative effects by disabling the atkbd_timeout timer.
I'm using a Lenovo Thinkpad T420.

I suspect it has to do with locking issues. There have been some attempts to
improve locking in atkbd and psm [3] but it turned out that this locking
approach could not be used [4].


[1]:
https://svnweb.freebsd.org/base/head/sys/dev/atkbdc/atkbd.c?revision=326255&view=markup#l161
[2]:
https://github.com/jiixyj/evdevfbsd/blob/e64ae7da580c9ccede482b380440d7b4636b4943/backend-atkbd.c#L108
[3]: https://lists.freebsd.org/pipermail/freebsd-arch/2017-May/018193.html
[4]: https://reviews.freebsd.org/D10263#221930

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